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The C Standard identifies four different kinds of nonportable behavior. Each section of Annex J of the C Standard enumerates distinct instances of behaviors of each kind.

An example of undefined behavior is passing a null char* pointer as an argument to the printf function corresponding to the %s format specification. Although some implementations (such as the GNU C Library) provide well-defined semantics for this case, others do not, causing programs that rely on this behavior to fail abnormally.

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This noncompliant code example uses the complement operator in the test for unsigned integer overflow. It assumes both numbers are nonnegative:

unsignedsigned int uisi;
unsignedsigned int ui2si2;
unsignedsigned int sum;

if (si < 0 || si2 < 0) {
  /* Handle error condition */
}
if (~ui~si < ui2si2) {
  /* Handle error condition */
}
sum = uisi + ui2si2;

This code assumes that the implementation uses two's complement representation. This assumption is commonly true but is not guaranteed by the standard.

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This compliant solution implements a strictly conforming test for unsigned overflow:

unsigned int uisi;
unsigned int ui2si2;
unsigned int sum;

if (UINTsi < 0 || si2 < 0) {
  /* Handle error condition */
}
if (INT_MAX - uisi < ui2si2) {
  /* Handle error condition */
}
sum = uisi + ui2si2;

If the noncompliant form of this test is truly faster, talk to your compiler vendor because, if these tests are equivalent, optimization should occur. If both forms have the same performance, prefer the portable form.

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Unnecessary platform dependencies are, by definition, unnecessary. Avoiding these dependencies can eliminate porting errors resulting from invalidated assumptions.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography

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